The Aedes mosquitoes, which are mostly responsible for passing on the virus to humans, will be floating around parts of southern Europe this summer, warned Prof Sam McConkey, head of the Department of International Health and Tropical Medicine at the Royal College of Surgeons.

"In the summer time, when the heat is up, those Aedes mosquitoes start breeding.

"Everyone is terrified of a big outbreak in southern Europe or the southern states of America," he said.

Currently, the spread of infection by mosquitoes is confined to countries in Central and South America as well as parts of the Caribbean.

However, Prof McConkey said these mosquitoes could also pose a threat in parts of the United States later this year as the Aedes species is common in the summer from Texas to Washington.

There is also a chance it could spread to parts of Asia, including China.

The Aedes mosquito is known as the Asian tiger and carries the disease.

It is more tolerant to cold than other mosquitoes.

He said the fear of an outbreak in Europe and the southern US States this summer is one of the main drivers behind the World Health Organisation declaring the spread of the virus an international emergency.

The virus is not life-threatening and clears up after a week.

But it poses a danger to the unborn children of pregnant women as it can cause the baby to be born with microcephaly, which affects brain development.

A man and a woman in an older age group, who are unrelated, tested positive for the virus after coming back to Ireland in recent months from South America but are now well recovered.

However, confirmation yesterday that the virus was passed on to a woman in Dallas in the United States through sexual transmission means that someone who never travels to an affected country could be at risk.

The confirmation of sexual transmission has been described as a "game changer" by health authorities and they are urging men who have travelled to an affected country not to engage in unprotected sex for six months if they are diagnosed with the virus.

Blood

Prof McConkey said most people who have the virus have no symptoms but they could have it for five to 10 days.

"To keep on the careful side, men having sex should wear condoms for a month after they have returned from an affected country," he said.

"If they have been diagnosed with the virus, they should wear condoms for six months."

He said there is a need for more solid data about all aspects of the virus, its spread and effects.

There is also a risk that it could be transmitted through blood.

Women who are pregnant are advised to avoid countries where there are outbreaks.

Meanwhile, the Irish Travel Agents Association (ITAA) are advising Irish citizens to take the appropriate precautions if visiting Central and South America and the Caribbean.

Chief executive Pat Dawson said member agents are working closely with their airline and hotelier colleagues to minimise any inconvenience and expense to their customers, particularly pregnant women, who may wish to change their travel plans to areas affected by the Zika virus outbreak.

"We will continue to monitor the situation in Central and South America and the Caribbean in close partnership with the Department of Foreign Affairs," said Mr Dawson.

"We recommend that all of our customers travelling to the affected areas become aware of the risks of Zika virus."

The Department of Foreign Affairs has issued travel advisories for 20 countries in response to the outbreak and are working with the HSE and Irish missions in the Americas to continually review this travel advice.